1. Field of the Invention
The present invention relates to a brushless motor, and in particular to a two-phase brushless DC motor, which produces rotation torque having a phase difference of 30 degrees with two-phase brushless motor driving current having 90 degrees phase difference to result in a reduction in the torque pulsation in a brushless motor and an improvement in the starting torque, and also increases a permeance coefficient of the rotor magnetic circuit over that of the brushless motor rotor having a phase difference of 60 degrees to thereby improve the efficiency of the motor.
2. Description of the Related Art
In general, a brushless motor refers to a DC motor which modifies a rectifier equipped with a brush as a mechanical part into an electric means.
Accordingly, it has been studied and developed in various fields because no wear, no dust, and no electric noise is produced and it is good for output and efficiency to make it adapted to high speed rotation type motor.
In this brushless motor, the rotor of the DC motor around which coils are wound is substituted with a permanent magnet, and the speed control method has been changed from a voltage control type into a magnetization phase control type to thereby require a driving circuit.
Further, the brushless motor in general comprises a rotor made of permanent magnet and a stator which is magnetized by rectified electric voltage of an electronic switching circuit.
The two-phase brushless motor is driven with the magnetization angle having 90 degrees phase difference, is comprised of a rotor made of permanent magnet having 2×n poles and a stator having 4×n pole windings.
Meanwhile, the three-phase brushless motor is driven with the magnetization angle having 60 degree phase difference, and is comprised of a rotor made of permanent magnet having 2×n poles and a stator having 6×n pole windings.
FIG. 1 is a view for showing rotation power transformation construction of a conventional two-phase brushless motor, and FIG. 2 is a diagram of a wave shape of a rotation torque for showing a driving step or operation of the conventional two-phase brushless motor.
As shown in FIG. 1, a conventional two-phase brushless motor comprises a basic rotor 2 with two poles and a stator 1 having four pole windings.
The thus constructed two-phase brushless motor produces rotation torque having 90 degree phase difference as shown in FIG. 2.
In this instance, as winding current flows between 0 and 180 degrees and big current flows between 0 and 45 degrees due to small counter electromotive forces, it is necessary to take note of a driving circuit and the angle of a torque ripple is big.
Furthermore, it is preferable to shorten the distance between poles of the permanent magnets in the rotor 2 and broaden a pole area of the stator 1 to correspond to that of the permanent magnet of the rotor 2 and decrease air gap defined there-between so that it is possible to make use of the magnetic energy to the maximum by increasing the permeance coefficient of the motor magnetic circuit.
However, since the two-phase brushless motor cannot but to be comprised of the rotor 2 having 2×n poles and a stator 1 having 4×n pole windings, the distance between poles of the permanent magnets in the rotor 2 is relatively large in comparison with that of the pole windings of the stator 1, and the pole area of the stator 1 corresponding to that of the permanent magnet of the rotor 2 is small to result in a small permeance coefficient and low utilization efficiency of the magnetic energy.
Therefore, according to thus two-phase brushless motor, it is not adaptable to a large size motor because it is great in torque ripple and small in electromotive torque.
FIG. 3 is a view for showing rotation power transformation construction of a conventional three-phase brushless motor, and FIG. 4 is a diagram showing a wave shape of a rotation torque for illustrating a driving step of the conventional three-phase brushless motor.
Meanwhile, as shown in FIG. 3, the conventional three-phase brushless motor is comprised of a basic rotor 20 with two poles and a stator 10 with 6 polar windings.
As shown in FIG. 4, the three-phase brushless motor produces rotation torque having 60 degrees phase difference, and winding current flows between 0 and 120 degree and a torque ripple angle having small counter electromotive force exists between 0 and 30 degree.
However, although the three-phase brushless motor is more advantageous than the two-phase brushless motor, since it cannot but to be comprised of the rotor 20 having 2×n poles and the stator 10 having 6×n pole windings in construction, the distance between poles of the permanent magnets in the rotor 20 is long, and the pole area of the stator corresponding to that of the permanent magnet of the rotor 20 is small to result in a small permeance coefficient and low utilization efficiency of the magnetic energy.
Although the two-phase or three-phase brushless motor is driven through or by a wave bipolar magnetization, which is a conventional optimum driving method, limitations have exist in utilizing the magnetic energy to the maximum in view of the principal structure, and structural problems arise in improving efficiencies.